This is a double-blind study designed to characterize the relationship between change in folic acid and change in total plasma homocysteine levels for modest changes in folic acid intake. The specific hypotheses are for significantly decreasing homocysteine associated with each increasing dose of folic acid in the range 0 to 400ug daily within each genotype of methylenetetrahydrofolate reductase (MTHFR). After screeening 1080 volunteers, 160 volunteers are expected to be enrolled in the study and, using a modified Latin Square design, they wil be randomly assigned to receive the 5 folic acid treatments, each lasting six weeks. Fasting venipuncture blood samples will be obtained at baseline and at the beginning and end of each treatment phase. Effects on total homocysteine levels will be compared between each treatment and dose response curves will be estimated.